1. Field of the Invention
This invention relates to an antenna module incorporated in a mobile wireless terminal or a wireless apparatus such as a mobile phone, personal data assistant (PDA), notebook computer, or a game machine, and to a receiving apparatus mounting the antenna module. Especially, this invention relates to the antenna module allowed to change a frequency characteristic of the antenna depending on a receiving frequency channel, and the receiving apparatus mounting the antenna module.
2. Description of the Related Art
A monopole antenna is proposed in Japanese Patent Publication JP-A-2001-251131, as a conventional antenna to be applied to a wireless system receiving a radio wave in a terrestrial analog broadcasting system or a terrestrial digital broadcasting system, in which a lower frequency than that of a mobile phone is used.
Such an antenna has a broadband characteristic and can be applied to the entire frequency band which is used in the system. However, for instance, since a wavelength received by the antenna is 64 cm in a terrestrial digital broadcasting system of Japan, there is a problem that the monopole antenna is difficult to be incorporated in a mobile terminal, such as a mobile phone, PDA, etc. receiving apparatus which have a maximum size of about 20 cm.
Presently, it is thought that the mobile phone will be combined with a receiving function for television broadcast or radio broadcast, in addition to a calling function. These broadcasting systems (i.e., television and radio) are assigned a different frequency band respectively and use a channel dividing the band by a frequency. For example, a terrestrial digital television broadcasting system of Japan is assigned a broad frequency band which is a UHF (Ultra High Frequency) band from 470 MHz to 770 MHz. Each broadcasting station is assigned a band, the bands being determined by dividing the broad frequency band by each 6 MHz (e.g., every 6 MHz). In such a system, a viewer needs to select a desired channel in order to watch a television broadcasting.
It is preferable that receiving the entire broad frequency band used by each wireless system is prevented, thereby to reduce an interfering wave (e.g., interference). Additionally, when an antenna size is sufficiently smaller than a wavelength of an electromagnetic wave adapted by the antenna (e.g., less than 1/10), an antenna having a narrower receiving frequency band is expected to have better antenna sensitivity. For the first reason, to prevent a signal of another wireless system, which is an interfering wave for the television broadcast, from being received, an unnecessary frequency channel other than a desired frequency channel is sometimes eliminated by, for example, a tracking filter circuit which is disposed in a subsequent stage of the antenna.
Meanwhile, since the mobile wireless terminal is required to be downsized for improving portability, the antenna is also required to be downsized. In view of the above, a wireless terminal including a tunable antenna in which a center wavelength matching an impedance thereof with a calling frequency can be controlled, is disclosed in Japanese Patent Publication JP-A-2003-298341.
When the tunable antenna which has a sufficiently smaller size than a wavelength of a received electric wave is used and incorporated into the mobile phone, a Q-value of the antenna is much higher than an external antenna (i.e., such as a rod antenna). Thus, when the tunable antenna is incorporated in the mobile phone and so on, a resonant frequency of the antenna is changed a few MHz by manual approximation (e.g., by hand). Since the antenna having a very high Q-value has a narrow band and a reactive power accumulated around the antenna is large, a resonant frequency of the antenna may be changed slightly by manual approximation to the antenna, so that the receiving characteristic of the antenna about the desired frequency may be degraded.
Additionally, since the tunable antenna includes a variable capacitative member therein, a characteristic variation of each component, such as the variable capacitive member (e.g., variable capacitance diode), cannot be prevented in a mass production. Thus, when the receiving apparatus is mass-produced by a cheap component having a large characteristic variation, it is difficult to keep a uniform characteristic in any receiving apparatus, and a yield is degraded thereby.
Thus, prior to the present invention, there has been no conventional system or method which can provide an antenna module or a receiving apparatus which can receive an entire radio wave band used in a terrestrial analog broadcasting system or a terrestrial digital broadcasting system, and which includes a small size antenna which can be incorporated in a mobile wireless terminal such as a mobile phone or a PDA, by controlling automatically the resonant frequency of the antenna which has a high Q-value, a narrow band, and a high sensitivity. Additionally, there has been no antenna module or receiving apparatus which can prevent the degradation of the receiving characteristic by manually touching a package and the degradation of yield by the characteristic variation of the component.